1. Field of The Invention
The present invention relates generally to a tape cassette including an erasure preventing plug for enabling or disabling a record mode of a playback/recording apparatus into which the tape cassette is inserted.
2. Description of The Prior Art
Recording/playback apparatus utilizing tape cassettes are well known in the art. A tape cassette to be utilized in such apparatus are generally provided with means to selectively enable or disable a record function of the apparatus to allow a user to protect recorded material from accidental erasure or to prevent program material from being recording over accidentally. One such means of enabling/disabling a record mode is providing a tape cassette with an erasure preventing plug. An erasure preventing plug is provided in the cassette body and is selectively movable, generally between two predetermined positions for enabling or disabling recording of the tape cassette.
The arrangement and functioning of a typical, conventional tape cassette erasure preventing plug will be explained hereinbelow with reference to FIGS. 7-9.
First, referring to FIG. 7, a tape cassette 1 includes an upper half 3 and a lower half 4. A circular opening 3h is defined by a generally cylindrical wall portion 3c extending downwardly into a portion of the lower half 4 to contact a circular opening 4h defined by a cylindrical wall portion 4c extending upwardly from the lower half 4. The bottom of the cylindrical wall portion 3c has a first projection 3p formed on one side on a lower surface thereof and the top off the cylindrical wall portion 4c has a second projection 4p formed on one side thereof, a bottom surface of which is vertically offset from the bottom surface of the first projection 3p. Thus, a substantially circular opening is formed through the cassette body, being comprised of opening 3h in communication with opening 4h and having a narrowed portion at a substantially middle portion thereof where the projections 3p and 4p are formed. An erasure preventing plug 5 is provided, rotatable arranged in the openings 3h, 4h such that a head portion 5a thereof is positioned in the opening 3h and a lower end of a leg portion 5d and lock projection 5e thereof is arranged in the opening 4h with a waist portion 5c and an upper part of the leg portion 5d passing through the narrowed portion between the openings 3h, 4h. As can be seen in the drawings, the erasure preventing plug 5 is biased in the upper direction by a coil spring 2 arranged between the top surfaces of the first and second projections 3p, 4p and the lower surface of the head 5a of the erasure preventing plug 5.
According to the arrangement shown in FIG. 7, a detector pin 6 of a video tape recorder, for example, does not contact the bottom surface of the lock projection 5e of the erasure preventing plug 5 and may project fully into the opening 4h, therefore recording on the tape cassette 1 is possible.
FIG. 8 shows an operation for changing the arrangement of the erasure preventing plug 5 so as to disable recording on the tape cassette 1.
Referring to FIG. 8, a screwdriver 7, or other such tool, is inserted into a slot 5r formed in the upper surface of the head 5a of the erasure preventing plug 5. Then, applying downward pressure against the head 5a of the erasure preventing plug 5 against the spring force of the spring 2, the waist 5c and leg 5d of the erasure preventing plug 5 are pushed downward and the lock projection 5e is separated from contact with the lower surface of the second projection 4p. Next, the screwdriver 7 is turned to rotate the head 5a of the erasure preventing plug 5 180.degree. via the slot 5r such that the lock projection 5e is rotated so as to project in the opposite direction than before rotation of the erasure preventing plug 5. Then, downward pressure on the erasure preventing plug 5 is released by removal of the screwdriver 7 and the spring 2 is active to push head 5a of the the erasure preventing plug 5 in the upward direction, bringing the lock projection 5e into contact with the first projection 3p which is disposed on a horizontal plane which is lower than that of the second projection 4p, thus upward movement of the erasure preventing plug 5 is restricted to less that that permitted before rotation thereof.
Referring now to FIG. 9, in this state, when the tape cassette 1 is inserted into the video tape recorder (VCR), for example, the detector pin 6 is contacted by the lower surface of the lock projection 5e and the VCR is prevented from entering a record mode and accidental erasure of program material recorded on the magnetic tape (not shown) of the tape cassette 1 is prevented.
However, according to this, as seen in FIG. 10, a certain amount of play Ar is present in the above-described arrangement. That is, a degree of lateral movement of the leg portion 5d may occur during adjustment of the erasure preventing plug 5 which may cause the lock projection 5e to become jammed on a side edge of the first projection 3p, for example. Thus, malfunction of the erasure preventing plug 5 may easily occur and reliability of the tape cassette is compromised. Further, during automated assembly of the tape cassette, the erasure preventing plug is held to the upper half while the upper half 3 is joined to the lower half 4, according to the above-described arrangement, the erasure preventing plug may easily become dislodged from the upper half 3 during assembly causing an increase in faulty assemblies during manufacturing.
For preventing these types of problems, an erasure preventing plug has been proposed which is disclosed in Japanese Utility Model application First Publication 62-12976, the arrangement of which is shown in FIG. 11.
As seen in the drawing, the erasure preventing plug 5 comprises a head 5a, including a slanted lower surface 5f, a waist 5c of a first diameter, a leg portion 5d of a second diameter, less that the first diameter, a lock projection 5e projecting substantially perpendicular to the axial direction of the leg portion 5d, and a slot 5r formed in the top surface of the head 5a for engaging a tool for rotating the erasure preventing plug 5. As may be seen in the drawing, a distance H1 present from the lower surface of the head 5a to the bottom of the leg 5d on a first side, from which the lock projection 5e is projected, is less that a distance H2 from the lower surface of the head 5a to the bottom of the leg 5d on a second side from which the lock projection 5e is not projected.
Operation of the above-described improved erasure preventing plug 5 will be explained hereinbelow with reference to FIGS. 12-16.
First, referring to FIG. 12, downward pressure is applied to the erasure preventing plug 5 in the direction of the arrow shown in the drawing, then, as seen in FIG. 13, as the slanted lower surface 5f of the head 5a compresses the coil spring 2, a large spring force is applied to the right side of the head 5a of the erasure preventing plug 5 than to the left. Therefore, in this `unengaged position` the plug lists slightly to the left, as seen in FIG. 13, and contact between the leg portion 5d and the projection 3p is prevented. Then, as seen in FIG. 14, the erasure preventing plug 5 is rotated approximately 180.degree., for effecting erasure prevention, or an `engaged position`, and, in FIG. 15, after rotation of the erasure preventing plug 5, in a compressed condition of the coil spring 2, a larger spring force is applied to the left side of the head 5a of the erasure preventing plug 5 than to the right, causing the pin to thus list to the right. According to this, pressure is maintained in the contact between the projection 3p and the lock projection 5e which can resist vibration and assure positioning of the erasure preventing plug 5. This feature facilitates automated assembly of the cassette.
However, there is a drawback in the above arrangement, as best seen in FIG. 16. That is, the coil spring 2 has a certain amount of play within the opening 3h in the direction of the arrow Arr of FIG. 16, in an unengaged position, similar to that of FIG. 13, the spring force exerted on the right side of the lower surface of the head 5a is greater than that exerted on the left side, this causes the coil spring to move to the extreme left of the opening 3h, pushed, as it were, by its own spring force. In this state an upper end 2u off the coil spring can extend beyond the periphery of the head 5a of the erasure preventing plug 5. If the erasure preventing plug 5 is operated in this condition separation of the end 2u of the spring and the head 5a off the erasure preventing plug 5 is worsened and the mechanical reliability of the mechanism is degraded by distortion of the coil spring 2 etc.
Thus, improvement of an erasure preventing plug mechanism for a tape cassette housing has been required.